1. Field of the Invention
The invention is in the field of metallic spectacle frames.
2. Description of the Prior Art
Nickel-bronze alloys of copper which can be strengthened or age-hardened by exposure to a temperature substantially below the melting point of the alloy are known in the prior art for use in such applications as bearings, valves, pumps and springs. The properties of these materials are discussed by Eash et al. in an article entitled "The Copper-Rich Alloys of the Copper-Nickel-Tin System" in the transactions of AIME, 1933, Vol. 104, pages 221-249, and by Wise et al. in an article entitled "Strength and Aging Characteristics of the Nickel Bronzes" in the transactions of the AIME, 1934, Vol. 111, pages 218-244 and additionally in U.S. Pat. Nos. 1,816,509 and 1,928,747.
The process of hardening such alloys by subjecting, for instance, castings to elevated temperatures such as at temperatures above 316.degree. C. over a period of about 5 hours is described as a means of "precipitation hardening" resulting in a change in the alpha domain in the alloy.
Recently, Plewes in Metallurgical Transactions, Vol. 6A for March 1975, pages 537-544, has found that prior cold work performed on copper-nickel-tin alloys affects the characteristics of such alloys which are subsequently age-hardened at elevated temperatures. Applications for such high strength copper based alloys disclosed by Plewes are connectors, diaphragm members and spring components in electromechanical relay packages. In work with a copper alloy containing 9 percent by weight nickel and 6 percent by weight tin, Plewes found that the minimum level of prior cold work required to effect the desired critical competitive balance between ductile/brittle properties is 75 percent reduction in area.
The majority of prior art spectacle frames are made of a non-heat-treatable or age-hardenable material such as pure nickel, Monel or the so called "nickel silver" which is an alloy of copper, nickel and zinc containing 10 to about 30 percent nickel and 5-33 percent zinc. In the process of producing the desired gage or diameter wire useful in such eyeglass frames, it is customary to cold work the wire alloy utilized, subjecting the alloy to a drawing operation to produce about 10 percent to about 75 percent reduction in area prior to assembling the frame in the desired shape and brazing the joints and other reinforcing parts of the frame. During the brazing operation which can be performed utilizing, for instance, electrical resistance heating, the spectacle wire frame is heated to a temperature of about 600.degree. C. to about 750.degree. C. to melt the brazing material utilized to effect the joint. In the process of heating the spectacle components, those components in the immediate vicinity of the joint tend to partially anneal with the result that the finished frame has weak spots at the brazed joints and marginal resistance to bending. As a result, the prior art frames have marginal resistance to bending and too easily acquire a permanent set subsequent to the application of stress to the frame such that the proper fit of said spectacles to the head of the wearer is not retained over a substantial portion of the life of the spectacle frame necessitating frequent readjustment of such frames. The fact that the metal spectacle frames of the prior art are made utilizing a material which cannot be increased in tensile strength or hardness subsequent to the brazing operation during assembly in which the frame is softened, results in a spectacle frame having weak areas specifically at the brazed areas.
It is known that beryllium copper alloys which also can contain nickel or cobalt have high strength and hardness and are useful as alloys in optical applications such as spectacle frames. A typical alloy analysis is as follows: beryllium 2.25 percent, nickel 0.35 percent and balance copper. Such alloys can be hardened by heat treatment but usage of such alloys in spectacle frames has been limited by the difficulty of successfully brazing such alloys in the assembly of spectacle frames because of beryllium content. The alloy tends to form a very adherent and refractory oxide of beryllium which makes the alloy difficult to pickle and braze as well as electroplate.
Heat-treatable alloys of Inconel and stainless steel of the 200 and 400 series are also subject to similar difficulties in the assembly of spectacle frames as noted above for beryllium copper alloys. In addition, the high yield points of these alloys make these alloys difficult to process and the annealing temperatures of these alloys are at least one hundred degrees centigrade higher than the nickel bronzes and thus more expensive annealing ovens would be required to utilize Inconel and stainless steel alloys.